As per history, I believe the US is the only nation that cannot be trusted with nuclear weapons.
Nobody can be trusted with nuclear weapons.
Britain would have nuked Germany to save itself if it had the opportunity and the necessity to do so, and or to bring the war to an end and kill Hitler if the opportunity were there. France too would have nuked Germany to defend itself and kill Hitler if possible. Had the Jewish people been able to, they too would not have blinked at nuking Germany and trying to kill Hitler in the process.
Japan would have nuked the United States and the rest of the allies. Germany would have nuked everybody at their leisure to 'win.'
The Empire of Japan was an extraordinarily powerful nation. Their military technology was very advanced, and they demonstrated endlessly that they were willing to use it brutally in instigation of war. They slaughtered millions upon millions in their Chinese invasion. It took a very substantial portion of America's considerable industrial base to defeat Japan.
It took two nuclear bombs before Japan capitulated with an unconditional surrender. The first one wasn't enough, which more than proves they were willing to shed millions more lives to keep fighting. America would have had to invade Japan and would have killed millions in taking the island to stop the war.
America had nukes before everybody else, and if their desire had been to do so, could have wiped out every other capital and brought the entire planet to its knees, regularly nuking anybody that dared to twitch about a nuclear program.
America also could have allowed a war to proceed with the USSR immediately after WW2, and nuked the USSR repeatedly and instantly become the sole superpower.
And in the last 67 years, America has specifically chosen to not use nuclear weapons of any sort, despite the radical military advantage that it has possessed for most of that time. Nukes in Vietnam would have ended that conflict very quickly.
It's a tragedy that Hiroshima and Nagasaki were nuked. Japan instigated war, both in general in the Pacific, and against America. Japan joined with Nazi Germany in a pact to destroy the allies. It was absolute war. I hope nobody reading this today ever has to really come to understand what that means.
Obviously the bombs killed fewer people than a full scale invasion of Japan (and from the perspective of the US, the only casulities were Japanese, not American and Japanese). But many historians would argue that a full scale invasion of mainland Japan was never going to happen after the USSR decided to engage Japan as well. The writing was on the wall.
That said, I think you have a good point using the counterfactual of what would have happend if anyone else got to the nuclear weapon first. Germany would have absolutely nuked anyone and everyone if they could. Japan might have as well (not sure what they would have used it against, maybe the Panama canal? I don't see how they could have launched one against mainland USA without an ICBM). The UK and USSR probably would have if they could.
Basically anyone that showed the stomach to firebomb entire cities would also have used the nuclear weapons of the day if they could.
It also took a couple of years before the full appreciation of the dangers of nuclear fallout became apparent. Not to mention, the nuclear weapons used in WW2 have a tiny yield in comparison to modern nuclear weapons.
On a point of detail, ironically it isn't true that 'the only casualties were Japanese." In Nagasaki there were a large number of conscripted foreign workers as well as the native Japanese population (mostly women and children).
Another good point about the firebombing. The US and UK airforce leaders were clear that if they lost the war the 1000 bomber raids onto civilian targets would likely be classified as war crimes and that they personally would be tried as war criminals.
They probably could have nuked Hawaii as well with some effort, and pushed the US forces even further back to shore and reduced our visibility / force projection.
For example during WW2 all of the major participants had chemical warfare weapons. Several, including Germany, had significant stockpiles. (Germany actually had the most effective stockpiles, though they seem to have not believed this at the time.)
The only significant use of chemical warfare was by the Japanese against other Asian countries that did not have chemical weapons. Nobody else dared use those weapons on each other for fear of the response.
And so it has remained. A lot of countries have chemical weapons. There have been a lot of wars between countries armed with them. There have been a number of threats that they would be used (for instance Iraq threatened to use them on Israel during the first Gulf war). And yet the only time they get used is against opponents who are not similarly armed. (For instance Iraq fought a bloody war against Iran without using chemical weapons - then used them on parts of its own population who they thought had been disloyal in the war.)
This fact gives me hope that we will continue to not use nuclear weapons as well.